THIS invention relates to electronic radio frequency (RF) identification systems. It more particularly relates to a transponder forming part of such a system, the system and associated methods of reading transponders and modulating an interrogation signal.
Electronic RF identification systems are well known in the art. Such a system comprises an interrogator or reader and a plurality of transponders. In use, the reader transmits an RF interrogation signal towards the transponders. In some systems, the transponders derive power to energize load circuits from the interrogation or energizing signal, The transponders then generate respective response signals comprising respective identification code data unique to the transponder, by respectively backscatter modulating the energizing signal with the respective identification code data. The reader reads the response signals in sequential manner, thereby to identify and/or count the transponders and/or articles with which they are associated, in use.
Each transponder comprises transponder circuitry presenting a load impedance to an antenna having an antenna impedance. In the known systems, the load impedance and antenna impedance are matched in known manner to yield optimum power transfer to the transponder circuitry, The applicant has found that especially in applications where the antenna apertures of a plurality of transponders overlap, these transponders are required to share the available energy and hence the transponders and the system do not always perform satisfactorily.
In WO 98/29760 in the name of Gemplus S.C.A. there is disclosed a system and transponder which do not utilize backscatter modulation of the interrogation signal and wherein certain transponders in the energizing field are selectively and deliberately detuned to alleviate the aforementioned problems. In other embodiments the impedance of the antenna and load impedance presented by the transponder circuitry are deliberately mismatched. In a preferred form of these embodiments, the load impedance is short-circuited when the transponder is not selected and not transmitting. A method of reading these transponders includes the extra steps of selecting and unselecting at least some of the transponders. Furthermore, the voltage recovery of these transponders when not selected may not be adequate in some applications.